This invention relates to a method and apparatus for supplying a sheet to a winding unit.
In the prior art shaft drive type sheet winding machine, the winding torque with which the rewind core shaft is driven for rotation is controlled to hold constant or progressively reduce the winding tension in the sheet during the growth of the wound sheet roll in order to obtain a high quality sheet roll.
The inventor has pointed out that the sole winding tension control as noted above is insufficient for precision winding and that control of the touch pressure with which the sheet is urged by a touch roller against the core is also important, and he has already proposed a specific arrangement to meet this requirement.
The inventor has been continuing studies and investigations concerning the method of supplying a sheet to a winding unit. In the prior art method, the sheet to be wound is withdrawn by the winding force of a core shaft, or the sheet is supplied at a speed, at which it is taken out from its supply roll, to a winding position. The supply roll of sheet, which is stored after it is formed, is usually very heavy, so that its sectional profile is liable to be changed from a true circle to an eccentric circle. When it is rewound, periodic fluctuations in the speed of the sheet being supplied occur, so that it is inevitable that the tension in the sheet fluctuates. A dancer roller is used to cope with the fluctuations in the sheet speed. However, the variation of the sheet tension is inevitable unless the mass of the dancer roller is zero so that the mechanical loss is zero. The variation in the tension in the sheet between the supply roll and the feed-out roller is carried past the feed-out roller to the following section of the sheet running path and constitutes a cause for variations in the sheet winding tension.
The fact described above was discovered by the inventor as a result of his pursuit of a high quality product. Heretofore, it has been believed that the winding tension is determined by its control through control of the core shaft drive torque and control of inter-sheet air layer through control of touch pressure alone, and the interest of engineers has been directed only to this aspect. It has been known that the variation in the tension results from eccentric rotation of the supply roll, but it has been considered that the variation is absorbed by the dancer roller so that it is only necessary to provide perfect control of the winding tension and winding touch pressure. However, if the supplied sheet already is under varying tension or has permanent strain, real improvement of the winding characteristics cannot be expected regardless of how precisely the winding tension and touch pressure are controlled between the core and the touch roller.
Particularly, with recent rapid progress of resin film techniques, there are being produced an increasing number of very thin films on the order of one micron thickness, for instance, and films which are very slippery or readily capable of elongation so that they are very inconvenient to handle. Also, there is a trend toward increasing the scale and operation speed of film production equipment, and wide supply rolls of 6 to 8 m have to be processed. Accordingly, a technique for taking out such a delicate and wide sheet from a supply roll having eccentricity and stably supplying it to a winding position has become very important.
The inventor first sought means for preventing the deterioration of the winding characteristics due to variation in the tension in the running sheet caused by the eccentric rotation of the supply roller. As a result, he contemplated once reducing the tension in the running sheet to zero immediately before the running sheet is wound on a core. This process was patented under Japanese Pat. No. 966,375.
Although this method is able to most reliably solve the problem of tension variation, it was subsequently found to be unsuitable for the control of the winding tension. More specifically, where a sheet perfectly free from tension is supplied between a touch roller and a core or a sheet roll growing thereon, in which case the friction between the sheet and the touch roller is zero, the necessary winding tension cannot be obtained unless the contact pressure between the touch roller and the sheet roll growing on the core is sufficiently high or a separate pinch roller cooperating with the touch roller is provided. A second drawback is that it is difficult to cause a sheet under zero tension to proceed in a correct posture to the winding position. Thirdly, it is undesirable to cause a sudden change in the sheet tension at the winding position. It is concluded that the sheet fed between the core of a shaft drive type winding unit and the touch roller must not only be free from tension variations but must also be under a tension adjusted to a level suited for the winding. The present invention is predicated on this conclusion.